Clutch issues!

hello everyone!
last year I started restoring my bike. I went through the carbs, wiring, did a steering stem bearing, and did a clutch. the clutch was slipping so I put new plates and discs in with springs. when I originally replaced the clutch I changed the oil to a non motorcycle oil. this year when I discovered it still slipped I put a new clutch hub in and springs along with a oil change with the correct oil. it still slips. I am out of ideas because everything has been replaced and the cable was adjusted well. it only slips under heavy load and the lever doesn't feel great, like it doesn't return correctly. any info would be great,

 

How long since you added the proper oil? Could be the non-motorcycle oil was so absorbed into the disks that it will take a while to get it out of the disks. Are you running synthetic or conventional oil? However, I would be concerned about the clutch lever not feeling right and not returning correctly.

 

If you replaced every but the clutch doesn't return correctly I'd said it's probably a clutch cable adjustment issue

 

Does the dot on the lever align with the line on the clutch cover?

 

Fix the obvious first - if the lever at the bars doesn't return it will always slip. So replace the cable.
And as @Franz (I think) said, make sure the lever is on the right spline at the clutch end. Once you adjust out the slack I'd be surprised if it slips then.